A solid oxide fuel cell (“SOFC”) is an electrochemical conversion device that produces electricity from oxidizing a fuel (e.g., hydrogen) at very high temperatures (typically 600° C. to 1000° C.). Each cell of an SOFC stack consists of a ceramic electrolyte positioned between an anode on one side and a cathode on the other. An oxidizing gas like air or oxygen passes over the cathode side, and when an oxygen molecule contacts the cathode/electrolyte interface, it catalytically acquires four electrons from the cathode and splits into two oxygen ions. The oxygen ions diffuse into the electrolyte and migrate to the other side of the cell (anode). The oxygen ions encounter fuel at the anode/electrolyte interface and react catalytically, producing water, heat, and two electrons (per oxygen ion). The electrons transport through the anode to the external circuit and back to the cathode. This provides a source of useful electrical power. Not all of the fuel is utilized, and so left-over fuel (“unspent fuel”), water vapor, and any other fuel byproducts exit the anode side of the fuel cell. The remaining oxygen, and inert portions of the air stream (e.g., nitrogen) exit the cathode side.
Highly efficient, low emission power systems may be based on this technology. The efficiency of an SOFC power system can be further augmented if it is paired with a bottoming cycle, such as a turbine-compressor-generator, to scavenge the high quality waste heat from the SOFC. This embodiment is referred to as a “hybrid-SOFC Power System”. The efficiency of such a system far exceeds that of an internal combustion engine (ICE), with drastically reduced emissions. The efficiency of a hybrid-SOFC power system also exceeds that of a proton exchange membrane fuel cell (PEMFC). Additionally, a hybrid-SOFC power system is a lighter option (i.e., has a higher energy density) than a battery.
SOFCs also have another advantage over PEMFCs and batteries as they can operate on heavy hydrocarbon fuels if the fuel is properly processed. Heavy hydrocarbon fuels, such as jet fuel, must be decomposed using a “reformer” to form products like carbon monoxide and hydrogen before use in a fuel cell. For a PEMFC, carbon monoxide is a poison to the anode, so an additional step is required to convert the carbon monoxide into carbon dioxide and hydrogen (water-gas shift reaction). SOFCs are better suited to heavy hydrocarbon fuels because the water-gas shift reaction occurs within the stack under equilibrium conditions. Additional hydrogen is thereby produced and is used as a fuel. In addition, heavy hydrocarbon fuels contain sulfur to varying degrees. Sulfur compounds are known to poison the catalytic activity of many metals, including anode materials of fuel cells. As a result, PEMFCs cannot withstand any sulfur in the fuel. SOFCs are more tolerant of sulfur, and processing of the fuel with a “desulfurizer” or utilizing an ultra-low sulfur fuel can reduce sulfur levels to acceptable levels (e.g., less than 10 ppmw). SOFCs can therefore offer very high efficiencies and low emissions, while also providing the ease of use of readily available hydrocarbon fuels.
SOFC stacks operate at temperatures of 600° C. to 1000° C. and are made up of several different materials that are chosen to ensure electrochemical performance while minimizing thermal expansion differences so that damage doesn't occur during thermal transients, like start-up. Since the materials are not a perfect thermal match, compliance in the stack is enabled by design tolerances and proper start-up procedures. Typical test stand stack heating rates are about 3° C. to 5° C. per minute, but could be as high as 30° C. per minute for a well-designed stack. Therefore, it could take anywhere from 30 minutes to well over an hour to heat up such a system. During this start-up period, little or no electrical power is produced, however.
Accordingly, there is a need for a hybrid-SOFC power system that overcomes the problems discussed above.